1. Field of the Invention
The present invention relates generally to solar cells and, more particularly, to a method and apparatus for the front surface metallization and encapsulation of solar cells.
2. The Prior Art
In the fabrication of solar cells, one of the costliest steps involves applying a metallized contact grid to the cell front surface. Some high-efficiency solar cells have vacuum-deposited metal grids as front contacts. In this case, the metal is either evaporated onto the cell front surface through a shadow mask or else is selectively removed by photolithography from a continuous coating on the cell front surface. Often, the thickness of the deposited metal layer must be increased by electroplating or solder-dipping. These vacuum-deposition techniques are expensive because of inherent low speed and complexity and also because they are characterized by inefficient use of metal.
Of late, much effort has been directed at screen-printed ("thick-film") metal and directly plated metal for the front contact metallization of solar cells. Neither of these processes is yet a fully developed technology in the manufacture of solar cells, although some solar cells with screen-printed ("thick-film") contacts have become commercially available. Furthermore, with such screen printing, narrow conductor widths are difficult to produce and conductor cross section is relatively flat. As a result, resistivity of the bulk metal is high, with specific contact resistance being on the order of 100 m.OMEGA.-cm.sup.2. Furthermore, in making screen-printed ("thick-film") metal contacts on the front surfaces of solar cells, the p-n barrier junctions must be formed relatively deep to avoid cell degradation by the "spiking" of metal through the cell junction during the firing operation required to produce good adhesion and low conductive resistivity. The directly plated metal process, on the other hand, is still in the experimental stage. Both of these processes are rather sensitive to surface contamination, further increasing their costs. The problem of achieving low cost front contact metallization of solar cells, therefore, is and remains a fundamental limitation to the widespread terrestrial use of solar energy as a viable source of low cost electricity.